1. Field of the Invention
The present invention relates to a method and apparatus for inspecting a film quality of a polysilicon film formed on a substrate of, for example, an LCD (Liquid Crystal Display).
2. Description of the Related Art
In the manufacturing of an LCD, a laser beam is often applied to an amorphous silicon film (hereinafter referred to as a-Si) film formed on a substrate of the LCD to thereby transform it into a poly-silicon (hereinafter referred to as p-Si) film, thus forming a semiconductor film having a high electron mobility. In this case, an excimer laser beam is applied to the a-Si film by the use of an ELA (Excimer Laser Anneal) method.
According to a process using this ELA method, the a-Si film is melted and crystallized instantaneously to give less thermal damages to the substrate, thus making it possible to form a p-Si film by a low temperature process at about 450° C. or less. This gives an advantage of being able to form the p-Si film using an inexpensive glass substrate with a large area.
The electron mobility is here indicated as μ=|vd/E|(cm2/SV), which gives a value/unit field of an average moving speed (drift speed: vd (cm/s)) of the electrons in crystal when an electric field E (V/cm) is applied to the crystal.
By using such a p-Si film, a TFT (Thin Film Transistor) with a high electron mobility can be formed on a glass substrate by a low temperature process. By using a p-Si film and a TFT, the problem can be solved to thereby obtain a thin and high-definition LCD of a so-called driver monolithic type in which a driver-section TFT and a picture-element section TFT are formed on a glass substrate.
When an a-Si film is transformed into a p-Si film through annealing by use of the laser beam, however, it is necessary to monitor whether it is formed into an appropriate crystal state (particulate diameter: 0.2–1.0 μm or more).
In monitoring of the crystal state, to check the crystal state the securest technology is to monitor the entire glass substrate directly through an optical microscope, which method however cannot possibly be applied to a field production line. To automate this method so that it may match the actual production line, there is available a technology disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-51078, which utilizes correlation between a crystallization ratio and a refractive index of a film to determine the refractive index using a semiconductor laser ellipsoid with a wavelength of 1294 nm, thus deciding whether a relevant product is to be accepted or rejected.
Furthermore, for example, Jpn. Pat. Appln. KOKAI Publication No. 3-97219 discloses a technology for, while applying an inspecting beam, optimizing the crystal state of the substrate to be processed, on the basis of a resultantly detected beam intensity.
Furthermore, Jpn. Pat. Appln. KOKAI Publication No. 6-244276 discloses a technology for, when detecting/evaluating a change in phase such as a crystal or amorphous state of a semiconductor film by applying energy beam, applying an annealing laser beam vertically to the semiconductor film to receive a resultant reflection beam spectrally so that a spectral reflectivity distribution thereof may be compared to data stored in a database, thus evaluating the crystallization state of the semiconductor film.
Furthermore, Jpn. Pat. Appln. KOKAI Publication No. 6-244255 discloses a technology for using a spectral profile of the reflection beam of a silicon film to thereby calculate its poly-crystallinity ratio and amorphous-ness ratio.
Furthermore, Jpn. Pat. Appln. KOKAI Publication No. 11-274093 discloses a technology for applying a processing laser beam and a measuring laser beam simultaneously to a silicon film to monitor their reflectivity, thus determining the layer state of the silicon film.
Furthermore, Jpn. Pat. Appln. KOKAI Publication No. 2000-133614 discloses a technology for applying the laser beam in pulse to a silicon layer to determine a state of the silicon layer based on a time-wise attenuating waveform.
Furthermore, Jpn. Pat. Appln. KOKAI Publication No. 11-121378 discloses a technology for measuring a reflectivity of a glass substrate on its back side on which a silicon layer is formed, to determine a state of the silicon layer based on the measurement of the reflectivity.
By the technology disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-51067 for optimizing the crystal state of the substrate to be processed only based on the intensity of the inspecting beam, however, the substrate cannot be heated sufficiently, so that a part thereof where the a-Si film is not melted can be known for sure but a part thereof where the a-Si film is melted but fine crystal particulate (particulate diameter: 0.01–0.02 μm approximately) transformed into a granular state owing to excessive heating, that is, a part where micro-crystal silicon (μ-cSi) is formed cannot be detected.
This part where the granular-state fine crystal particulate is formed has an extremely low average electron mobility, not higher than a value of 100 (cm2/S·V) supposed to be appropriate in formation of a driver section TFT. By this method, therefore, it is impossible to know whether a p-Si film is formed uniformly, thus possibly deteriorating the yield of manufacturing the TFT.
Furthermore, by the technology disclosed in Jpn. Pat. Appln. KOKAI Publication No. 3-97219 for applying a visible reference laser beam to a site where the excimer laser beam is being applied to evaluate a crystallization level of the site based on a change in intensity of the resultant transmission or reflection beam, there is a problem that it is impossible to obtain information of re-crystallization after actual poly-crystallization.
Furthermore, by this method, the excimer laser beam is applied in pulse by a few shots, so that when the visible reference laser beam is applied, its transmission or reflection beam changes in intensity, thus giving rise to a problem of difficulty to decide crystallinity. Besides, there is a problem of complexity of an apparatus for electrically processing and detecting the time-wise changing intensity of the transmission or reflection beam.
Furthermore, by the technology disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-51078 for measurement by use of a semiconductor laser ellipsoid, it is necessary to know an accurate film thickness beforehand for calculating the refractive index. Besides, calculation thereof takes much time, thus deteriorating the throughput. Moreover, an optical system for the measurement apparatus becomes large and complicated.
Furthermore, by the technology disclosed in Jpn. Pat. Appln. KOKAI Publication No. 6-224276, the annealing laser beam is applied vertically to a semiconductor film to receive its reflection beam spectrally in order to then compare its spectral reflectivity distribution to the data stored in a database and evaluate a crystallization state of the semiconductor film, so that it is necessary to measure the spectral reflectivity of the semiconductor film for each waveform and also create the data of its consecutive profiles, thus suffering from complex data processing. This problem holds true also with the technology disclosed in Jpn. Pat. Appln. KOKAI Publication No. 6-244255.
Furthermore, by the technology disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-274093, the processing laser beam and the measuring laser beam are applied simultaneously to a silicon film to monitor the reflectivity thereof in order to detect a layer state of the silicon film, so that the reflectivity of the processing laser beam and that of the measuring laser beam are added to each other to give rise to mutual interference of the beam, thus disqualifying the method for use in accurate measurement.
Furthermore, by the technology disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-133614, the laser beam is applied in pulse to a silicon layer to determine a state of the silicon layer based on the shape of the time-wise attenuating waveform, thus complicating the processing of data such as waveform data.
Furthermore, by the technology disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-121378, reflectivity of a glass substrate is measured on its back side on which a silicon layer is formed, to determine a state of the silicon based on the measurement, thus making it impossible to directly determine the surface state of the silicon layer itself.
In view of the above, it is an object of the present invention to provide a film quality inspecting method and apparatus which can inspect a crystal state of a silicon substrate speedily and simply.